Rust is a fungus. It is commonly found in the Southwest and along the Pacific Coast. It requires moisture to develop and is most often seen in the late summer and early fall or during periods of frequent rain and temperatures above 65 degrees F.Symptoms: Initially, rust appears as small, powdery, orange spots on the underside of leaves. It can develop into larger orange and brown spots on the upper surfaces.Treatment: Remove diseased leaves both from the plant and the soil at its base. There are a number of fungicides that can combat this fungus.